The present invention relates to a DELTA-SIGMA (hereinafter referred to as xcex94-xcexa3) modulator and a xcex94-xcexa3 A/D converter having the xcex94-xcexa3 modulator. The present invention is applicable to ultra-high communication apparatuses such as software radio and high-frequency measuring apparatuses.
An A/D (analog-digital) converter is a circuit having a function for converting an analog signal into a digital signal and indispensable as an interface between an actual world indicated by analog information and a digital signal processing circuit.
As the conventional A/D converter, there are mainly two types of A/D converter such as the flash type and the xcex94-xcexa3 type. In the flash-type A/D converter, an input voltage is divided by means of many resistances, and divided voltages are applied in parallel to comparators to convert an input analog value to a digital value. Therefore, the flash-type A/D converter is capable of operating at a very high speed but incapable of coping with variations of characteristics of elements and has difficulty in obtaining high precision (number of bits).
On the other hand, in the xcex94-xcexa3 A/D converter, an input signal is converted into a digital signal of low bit precision at a very high sampling rate. Then a signal having a required frequency is taken out from the digital signal and then high-precision digital data is obtained by converting precision in time base direction into precision in bit. The circuit is capable of coping with variation in a device parameter and making high-precision conversion. The operation principle of the xcex94-xcexa3 A/D converter is described below based on a document (xe2x80x9cA Simple Approach to Digital Signal Processingxe2x80x9d written by C. Marven and G. Ewers; translated by Hiroshi Yamaguchi, published by Maruzen). FIG. 1 shows a block diagram of the xcex94-xcexa3 A/D converter.
The xcex94-xcexa3 A/D converter comprises a xcex94-xcexa3 modulator (xcex94-xcexa3 quantizer) and a digital filter. The xcex94-xcexa3 modulator samples an input signal at a very high frequency and coverts the sampled input signal into a pulse density signal. The digital filter decreases a high-frequency component of the signal and converts precision in time base direction into precision in bit to thereby take out a digital signal having high precision.
FIG. 2 shows the simplest construction of the xcex94-xcexa3 modulator which is the main part of the xcex94-xcexa3 A/D converter. This circuit comprises an integrator and a comparator. This circuit integrates the deviation between an input signal and an output signal and controls an output in such a way as to minimize the deviation. The output signal becomes a pulse signal. As a result, the pulse density is so controlled as to be coincident with an input value, and a pulse density signal proportional to the input value is obtained. At this time, by setting the frequency of a clock signal for sampling much higher than a Nyquist frequency, the A/D converter is capable of obtaining high precision.
As described above, because this circuit has high precision by increasing the sampling rate, the circuit of the xcex94-xcexa3 A/D converter has advantages that it is capable of coping with variations of a circuit parameter than the flash-type A/D converter and does not require a high-precision analog element in obtaining high precision.
In the xcex94-xcexa3 A/D converter having the above-described characteristics, a very high sampling frequency is necessary for a frequency of a signal to be obtained. Thus the conventional art constructed of a transistor, an operational amplifier, and the like has a problem that it can be used only for an audio signal having a comparatively low frequency or the like.
It is an object of the present invention to provide a novel xcex94-xcexa3 modulator which has overcome the problem of the conventional art and a xcex94-xcexa3 A/D converter using the xcex94-xcexa3 modulator.
The xcex94-xcexa3 modulator according to the present invention has a comparator having a plurality of negative resistance elements which show an N-type differential negative resistance and are connected to each other in series between two terminals and a comparison input transistor connected in parallel to at least one of the negative resistance elements, a clock pulse voltage being applied between the two terminals; a conversion input transistor for converting an input voltage into electric current; a capacitance element connected to an output current terminal of the conversion input transistor and to an input terminal of the comparison input transistor of the comparator; and a feedback transistor to which an output of the comparator is inputted and through which electric current flows into the capacitance element in a direction opposite to a direction of electric current flowing through the conversion input transistor. The xcex94-xcexa3 modulator is capable of achieving a high-speed operation, unlike the conventional one.
The above-described negative resistance element can be a resonant tunnel element such as a resonant tunnel diode or other elements showing a differential negative resistance such as an Esaki diode. The transistor can be a field effect transistor, a bipolar transistor or the like.
Further, the present invention is characterized in that the xcex94-xcexa3 A/D converter comprises the above-described xcex94-xcexa3 modulator and a digital filter. According to this invention, it is possible to realize a high-precision A/D converter operating at an ultra-high frequency.